Sound-recording method



Patented July 26', 1921.

UNITED STATES PATENT OFFICE.

JOSEPH P. ummnn, or wromme, New messy, assronon, 3r Assmumrs, T wns'rnnnELECTRIC conramr, mcoarona'rnn. A conrom'rron or NEW YORK.

souNn-naconnme 'mnrnon.

Application filed January 17, 1925. Serial No. 2,971.

This invention relates to sound recording and particularly to methods ofproducing phonograph sound records of the rotatable disc type.

5 An object of the invention is a method for producing phonograph soundrecords which will give a greatlyincreased duration of effective run fora given size of record.

As is well known, the existing methods for producing sound recordsusually include as one step in the operation the use of a cutting toolor stylus for cutting a groove in a suitable moving surface, thiscutting tool or stylus by mechanical connection to a diaphragm or byother means being adapted to vibrate either vertically or horizontally'in accordance with sound waves representing the sounds to be recorded,thus producing in the groove undulations corresponding to the soundvariations.

This invention has specialapplication to phonograph records of therotatable disc type in which sound variations are recorded on thesurface over a region extending from the circumference to a regionnearer the center of the record. The applicant has discovered that thereis a definite relation existing between the time such a record will run,the speed of rotation of the record, its diameter, and the particularportion of the sur- I face of the record utilized for recordingpurposes, such that by properly selecting these latter elements it ispossible to increase the effective duration of run for a given size ofrecord greatly beyond that obtainable in records produced methods of theprior art. This is especlally important in con-. nection with theso-called talking motion pictures and the recording of long symphonyorchestra programs where it is desirable toemploy phonograph soundrecords which will run a considerably longer time than records of theprior art. This result was arrived at by the following mathe maticalanalysis:

The length of time for which a given record will run depends upon thefollowing factors:

(1) The outside diameter oif the record.

(2) The slowest linear speed at which sounds can be recordedsuccessfully.

*(3) The number of threads or grooves cut per inch of radius.

(4) The angular velocity at which the record disc is rotated.

The slowest linear speed at which sounds can be recorded successfullydepends upon the highest frequency to be recorded and the diameter ofthe needle which is to reproduce these sounds. It has been foundexperimentally that with records recording all sounds up to frequenciesof 6,000 vibrations per second and which are to be reproduced with thepresent commercial steel needles, the lowest value of linear speed is ofthe order of 15" per second.

The relationship existing between the above four factors can best beshown by the following equations:

, Let R=the radius of the outside of the recording cut on the disc ininches.

r=the radius of the inside of the recordin cut on the disc in inches.

=the number of threads or grooves cut per inch of radius.

V =the minimum linear speed in inches per second at which successfusound recordlng can be effected.

s=the angular velocity of rotation of the disc in radians per second.

t=the duration of run of the record in seconds.

t =the maximum duration of run for a record of given size in seconds.

s =the angular velocit of the record corresponding to the time 0 run tT=the duration of run of the record in minutes.

T =the maximum duration of run for a record of given size in minutes.

S=the speed of rotation of .the disc in revolutions per minute.

S =the speed of rotation of the disc in revolutions per minutecorresponding to the time of run T A=total angle through which the discwill rotate in the time t.

Since the minimum velocity occurs at the inside of the record, that is,where the radius equals 1, V 8r (1) The total number of revolutionswhich the record will make is equal to (R1')l l and, therefore, thetotal angle through which the record rotates will be represented byA=21rN (B -7') (2) 2.N vo t== 8 R8-) (5) Solving equation (5) for amaximum value of t, it is found that t isamaximum when the angularvelocity is given by:

Substituting this value of 8 for 8 in equation (1), it is found thatwhen t is a manmum,

Also

Substituting the value of a of e uation (6) for s in e uation (5) or theva ue of r of equation in equation (4) a maximum value for t isobtained.

Eliminating R from equations (6) and (8),

the following equation is obtained:

t s =21NV =a constant (9) For the sake of simplicity, equations (5),(6), (8) and (9) may be rewritten expressing angular velocity inrevolutions per minute instead of radians per second and time in minutesinstead of seconds. They then become:

, The above analysis applies to both the lateral and vertical type ofcut.

From the above mathematical analysis, it is apparent that the maximumduration of elfectlve run for a given sized record will be obtained ifthe portion of the surface of the record used for recording purposes isso selected that the distance of the inside of the recording cut issubstantially one half that of the outside of the recording cut from hecenter of the record, and if the speed of recording in revolutions perminute is made substantially equal to 60 times the minimum linear speedin inches per second at which successful sound recording can be effectedin the given record material divided by 1r times the distance in inchesof the outside of the recording cut from the center of the record.

The objects of the invention will be clear from the following detaileddescription read in connection with the accompanying drawing in whichFig. 1 illustrates the location of the recording cut as embodied in theinvention on the surface of a phonograph record of the disc rotatabletype, and Fig. 2shows a series of curves giving the relation betweenrecording speed and duration of effective run for disc phonographrecords of various diameters.

Referring to the drawing, in Fig. 1 is shown schematically in a recordof the disc rotatable type the relative distances from the center of thedisc of the outside 1 and the inside 2 of the recording cut, as embodiedin the invention. As shown in the drawing, the radius of the inside 2 ofthe recording cut is e ual to one-half of the radius R of the outside 1of the recording cut.

Referring to Fig. 2, a series of curves 3 is shown giving the relationbetween the speed of recording in revolutions per minute and theduration of run in minutes for records of various diameters ranging froma diameter of 10 inches to a diameter of-20 inches. These curves wereplotted in accordance with equation (10),

derived above. In accordance with standard phonograph practice, theconstants used are the following:

N 100 grooves per inch V 1.7 inches per second R d1ameter inches.

Referring to the curves 3, it should be noted that when the speed ishigher than that corresponding to T a smaller portion of the record iscut at speeds approaching the minimum allowable. Therefore, it isapparent that if the speed of recording to be used is not thatcorresponding exactly to the maximum time of'run, it is preferable tohave this speed slightly higher rather than lower than the proper one.

In Fig. 2, acurve 4 joining the maxima of the time curves 3 is alsoshown, this curve corresponding to equation (13) derived above.Referring to this curve, it should be noted that the upper part of thecurve is very steep showing that for records of large diameters, anaverage recording speed can be selected for several records of differentdiameters, which value of speed will give a time of run for each recordclose enough to the maximum time of runfor all practical purposes. Forinstance, records 16 inches in diameter and greater cut at a speed of 40R. P. M., a value just about one-half of the standard phonograph speedof the prior art, indicated by the dotted line in Fig. 2, will have agreatly incieased duration of run over records cut in accordance withthe standard practice of the prior art. From the curves 3, it will benoted that for a record 16 inches in diameter, the gain in duration ofrun resulting from operating at a speed of 40 R. P. M. instead of 80 R.P. M. will be 2.3 minutes out of 7.2 minutes standard run, or 32percent, for a record 18 inches in diameter 3.5 minutes out of 8.5minutes standard run, or 41 percent, and for a record 20 inches indiameter 4.9 minutes out of 9.7 minutes standard run, or percent. Arecord having a sound circle 16.2 inches in diameter, having itsrecording range limited in accordance with the invention and out and oerated at a speed of approximately 40 R. M. with 112 threads per inchwould have an effective duration of run sufficiently long to accompany1,000 feet of motion picture film, assuming the film to run at a rate of20 pictures per second. On the other hand, a record cut in accordancewith standard practice of the prior art and operated at standardphonograph speed would have to have asound circle whose diameter is 24.9inches in order to do this.

Referring to equation (11) above, 1t w1ll be noted that 3..., theangular speed of rotation corresponding to the maximum duration of run,is directly proportional to V,,,

the minimum linear speed at which successful sound recording can beeffected. As stated above, manufacturing conditions at the present timelimit V to about 16.7 inches per second and the, curves of Fig. 2 havebeen plotted using that value, but it is to be understood that it isnot-desired to set any limitations to the particular minimum linearspeed that may be used in practice.

By the term effective duration of run as used in the specification andclaims is meant the interval of time during which the recorded soundscan be reproduced.

.in or other well known been described above as applied directly to thepermanent sound records themselves, it

is to be understood,- of course, that the invention is also applicableto master sound reoords and to all permanent sound records made fromsuch master records by stampprocesses. Vhat is claimed is:

1. A method of increasing the time of effective run of a phonographrecord which comprises cutting the sound variations into a record blankat a speed in revolutions per minute substantially equal to times theminimum linear speed at which successful sound recording can be effectedin a given recording material divided by 1r times the radius in inchesof the outside of the recording cut from the center of the record, theradius of the inside recording out being limited thereby to,substantially one-half the radius of the outside recording cut.

2. A'sound record comprising a surface capable of receiving animpression and a groove on said surface, said groove having anundulatory contour corresponding to sound variations, said groove beingin a spiral having an inner radius substantially one-half the outerradius, said undulatory contour of said groove being of such characteras would be produced at such a constant speed of rotation of saidsurface that the linear speed at said inner radius is the minimum atwhich successful sound recordmg can be done in said surface.

3. A sound record comprising a surface capable of receiving animpression anda groove in said surface, said groove having undulationscorresponding to sound variations, and of such character that the normalsounds are reproduced at a record speed "in revolutions per minute of 60times the minimum linear speed in inches per mcond at which said soundvariations can be effective 1y recorded in said surface divided by 1rtimes the greatest distance in inches of said groove from the center ofsaid surface.

4..A sound recordcomprising a surface capable-of receiving an impressionand a groove 1n said surface, said groove having undulationscorresponding to sound variations, said groove being included in a ringhaving aninner radius and an outer radius, said inner radius beinsubstantially onehalf the outer radius, said undulations being of suchcharacter that the normal sounds are reproduced at a record speed inrevolutions .per mlnute substantially equal to 60 times circle ofmaximum diameter at least as small as 16% inches and having recordedtherein the sounds accompanyingthe action recorded upon 1,000 feet ofstandard motion picture film.

7. A sound record comprising a surface capable of receiving animpression and a groove in said surface, said groove having anundulatory contour corresponding to sound variations, said groove beingin a spiral having an inner radius substantially one-half the outerradius, said undulatory contour of said groove being of such characterthat the normal sounds are reproduced at a record speed in revolutionsper minute substantially 60 times the minimum linear speed in inches persecond at which sound variations can beefi'ectively recorded in saidsurface divided by 1r times the length in inches of said outer radius ofsaid spiral, said outer radius being such that the effectivesound-producing time of run of-said record will be substantially equaltofthat of 1,000 feet of motion picture film*moving at the normal rateof speed.

8.A sound record comprising a surfacecapable of receiving an impressionand a groove in said surface having an undulatory contour correspondingto sound variations, said groove being in a spiral having an innerradius substantially one-half of the outer radius of said spiral fromthe center of said surface, said undulatory contour being of suchcharacter that the normal sounds are reproduced at a record speed ofsubstantially 36 to 40 revolutions per minute, said outer radius of saidspiral being of such length that the effective time of run of saidrecord will be substantially equal to the time of 'run of 1,000 feet ofmotion picture film operated at a speed of twenty pictures per second.

In witness whereof, I hereunto subscribe my name this 16th day ofJanuary A. D., 1925.

JOSEPH P. MAXFIELD.

